Stabilization of organic isocyanates



2,885,425 V STABILIZATION OF ORGANIC ISOCYANATES Louis Spiegler,Woodbury, NIL, assignor to E. I. du Pont de Nemours and Company,Wilmington, Del., a corporation of Delaware No Drawing. ApplicationJanuary 27, 1956 Serial No. 561,914

4 Claims. (Cl. 260-453) Thisinvention relates to organic compounds, andmore particularlyto the stabilization of organic isocyanates againstdiscoloration.

5 ,Organic isocyanates, which in general are colorless liquids orsolids, tend to become discolored on storage. OIlTdlSCOIOI'alIiQD theseorganic isocyanates change to from light yellow to brown in extremecases, and while this color can be removed by distilling theisocyanates, the distilled products again become discolored on standingeven in only a few hours.

The discoloration of these organic isocyanates presents a seriousproblem where colorless or very light colored reaction products are tobe produced, for it is usually impractical to redistill the isocyanateimmediately before use.

The exact cause of the color formation in the isocyanates is not known,for it has been found that when these products are stored in the lightor in the dark even under nitrogen, the discoloration developspractically as fast as when the same products are stored in air. Thepresence of moisture is believed to be a factor in causing thisdiscoloration, but, again, it is impractical to package isocyanates,particularly when used in large commercial quantities, under absolutelymoistureproof conditions.

It is therefore an object of the present invention to provide a simpleand positive method for stabilizing organic isocyanates againstdiscoloration. It is a. further object of the invention to produceorganic isocyanates which for extended periods of time do not becomediscolored even in the presence of oxygen, moisture or light.

The present invention comprises the stabilization of organic isocyanatesagainst discoloration by incorporating in the organic isocyanate from0.01% to 0.5% of an essentially colorless organic disulfide which isessentially non-reactive with organic isocyanates under ordinaryatmospheric conditions, or reacts very slowly in the absence ofcatalysts.

These stabilizing agents are soluble in the organic isocyanates, whichare normally liquids and may be readily dissolved in the melted organicisocyanates which at normal temperatures are solids. While ordinarilythe amount of stabilizing agent employed will vary from 0.01% to 0.5based on the weight of the isocyanate to be stabilized, variousstabilizers hereinafter listed can be used in lesser amounts. Largeramounts of any of the stabilizers of course may be used up to theirlimit of solubility in the isocyanate, but unnecessarily large amountsof course are to be avoided since they merely constitute an addedimpurity in the product. When the very small amounts are used, theeffect of this impurity on the product is negligible.

The following list of alkyl disulfides and aryl disulfides of thebenzene series, when tested in the concentrations within the limits ofthose specified above and under the conditions given in the example,eifectively stabilize organic isocyanates against discoloration even inthe presence of oxygen, moisture or light. The parts and .2 vpercentages employed in this specification are by weight, unlessotherwise specified.

Example Toluene-2,4-diisocyanate is distilled at 4 to 5 of mercurypressure and the colorless fraction distilling at 106 to 108 C. iscollected in a four unit; multiple receiver, about 45 g. being collectedin each. The system is raised to atmospheric pressure by admitting drynitrogen. The desired amounts of stabilizers are added to thediisocyanate in three of the receivers under an atmos phere of nitrogen,while the toluene-2,4-diisocyanate in the fourth receiver isused as acontrol.

' Each of the four samples isthen divided, again under nitrogenatmosphere, into three portions in screw cap bottles. The cap of thefirst bottle is screwed on tightly and taped and stored over. 96%sulfuric acid. The second and third bottles are stoppered loosely sothat the ambient atmosphere can come in contact with the sample, thesecond being stored at roomtemperature at 20% 7 relative humidity whilethe third is allowed to stand in ordinary room atmosphere. Theunstabilized toluene- 2,4-diisocyanate bottles in both the 20% relativehumidity and at room conditions turn light yellow in a few hours and atthe end of hours exposure are brown colored. The stabilized samples inboth the 20% relative humidity and in ordinary atmosphere remainedcolorless at the end of 100 hours exposure.

Concentration in percent Stabilizer The following compounds furtherexemplify the class of organic disulfides which may be substituted forthose specifically used in the example and which will eifectivelystabilize the organic isocyanates against discoloration: methyldisulfide, ethyl-tert.butyldisulfide, allyldisulfide, n butyldisulfide,n-amyldisulfide, p-cyanophenyldisulfide, andgamma,gamma-dithiodibutyronitrile.

The stabilizing effect on solid isocyanates is shown when thestabilizing agents of this invention are added to them in molten form,for example, when one of the stabilizing agents is added to pure2,4,4'-triisocyanatodiphenylether, the mixture solidified and broken upunder nitrogen, and then exposed under the same conditions as thosegiven in the example.

Mixtures of isocyanates may of course be stabilized in the same manneras the individual isocyanates.

Any of the aromatic, aliphatic, cycloaliphatic, mono-, di-, or higherpoly-isocyanates which tend to discolor due to the presence of moistureor oxygen, may be eflfectively stabilized against discoloration by theaddition of the organic disulfides above described. As illustrative ofthe various classes of diisocyanates which may :be stabilized againstdiscoloration the following list [is given, any of which whensubstituted for the toluene-2,4-diisocyanate of the example, arestabilized against discoloration when tested according to the proceduredescribed therein:

Toluene-2,6-diisocyanate 4,4-methylenediphenylisocyanate4,4'-methylenedi-ortho-tolylisocyanate 2,4,4'-triisocyanatodiphenyletherToluene-2,4,6-triisocyanate 1-methoxy-2,4,6-benzenetriisocyanateMeta-phenylenediisocyanate 4-chloro-meta-phenylenediisocyanate 4,4biphenyldiisocyanate 1,5 -naphthalenediisocyanatel,4-cyclohexanediisocyanate 4,47-methylene-bis(cyelohexylisocyanate) I1,5-tetrahydror1aphthalenediisocyanatev Alpha' andbeta-naphthyleneisocyanate 1 ,4-tetramethylenediisocyanate 1,6hexamethylenediisocyanate I 1,10-decarnethylenediisocyanate Ortho-,metaor para-tolueneisocyanatc 4 -methoxy-rneta-phenylenediisocyanate' IThe conditions given in the example under which the above stabilizersare'tested are quite severe when it is considered that compounds such asorganic isocyanates' 2. An organic 'isocyanate of the class consistingof aliphatic, cycloaliphatic and aromatic isocyanates, stabilizedagainst discoloration by having dissolved therein from 0.01% to 0.5% byWeight, based on the isocyanate, of propyldisulfide.

3. An organic isocyanate ofthe class consisting of I I aliphatic,cycloaliphatic' and aromatic isocyanates stabia're ordinarily shipped insealed containers out of: contact with atmospheric moistureand oxygen,and often light.

When such products are stabilized with the compounds moisture and air. 1

I claim: I I I I '1. An organic isocyanate of the class consisting ofaliphatic, cycloaliphatic and aromatic isocyanatesstabiof an essentiallycolorless organic disulfide of the group more particularly describedabove and sealed in iron 1 drums, the efiectiveness of the stabih'zerwilly-be many times that given for tests carried out in the presence oflized against discoloration by having dissolved therein from 0.0l%to'0.5% by weight, based on'the'isocyanate, 25,

Y lize'd' against discoloration by' having dissolved, therein 7 I I from0.01%' to 0.5% by weight, based on'the isocyanate,

of his (2,4-dichlorophenyl disulfide.

:4; 'An organic lisocyanate' of the class consisting of I I I aliphatic,cycloaliphatic andaromatic'isocyanates stabilize'd' againstdiscoloration by having dissolved therein from 0.01% to 0.5% by weight,based on the isocyanate, I I

of ditertbutyldisulfide.

References Cited inthe file of this patent UNITED STATES PATENTS a"consisting'of aliphatic disulfides and aromatic disulfides Iofthebenzene series.

Rosen Oct. 8, 194 0

1. AN ORGANIC ISOCYANATE OF THE CLASS CONSISTING OF ALIPHATIC,CYCLOALIPHATIC AND AROMATIC ISOCYANATES STABILIZED AGAINST DISCOLORATIONBY HAVING DISSOLVED THEREIN FROM 0.01% TO 0.5% BY WEIGHT, BASED ON THEISOCYANATE, OF OF AN ESSENTIALLY COLOELESS ORGANIC DISULFIDE OF THEGROUP CONSISTING OF ALIPHATIC DISULFIDES AND AROMATIC DISULFIDES OF THEBENZENE SERIES.